The present invention relates to the field of optical fibers, and more particularly to the field of dispersion-shifted optical fibers serving for use in wavelength division multiplexed (WDM) transmission systems.
Single-mode optical fibers that are referred to as xe2x80x9cdispersion-shifted fibersxe2x80x9d (DSFs) are such that, at the transmission wavelength at which they are used (which wavelength is, in general, different from the wavelength of 1.3 xcexcm for which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero; i.e. the non-zero chromatic dispersion of silica is compensated (hence, the use of the term xe2x80x9cshiftedxe2x80x9d) by increasing the index difference An between the core of the fiber and the optical cladding. This index difference makes it possible to shift the wavelength for which the chromatic dispersion is zero. It is obtained by inserting dopants into the fiber while it is being manufactured, e.g. by a modified chemical vapor deposition (MCVD) process that is known per se, and that is not described in any more detail herein. The terms xe2x80x9ccladdingxe2x80x9d and xe2x80x9ccorexe2x80x9d are known to the person skilled in the art; conventionally, the xe2x80x9ccladdingxe2x80x9d is the portion that extends to a diameter of 125 xcexcm. The core corresponds to the portion in which about 70% of the light energy propagates.
As is well known to the person skilled in the art, DSFs differ in general from conventional stepped-index fibers that are referred to as xe2x80x9cnon-dispersion-shifted fibersxe2x80x9d (NDSFs), and they also differ from xe2x80x9cdispersion-flattened fibersxe2x80x9d which have low chromatic dispersion over a wavelength range 1310 nm to 1550 nm, and which are such that they generally have two chromatic dispersion zeros in the range in question.
Numerous index profiles have been proposed for such dispersion-shifted single-mode optical fibers. The index profile is generally described by the curve representing the variation in the refractive index as a function of the radius of the fiber. Conventionally, the x-axis gives the distance r to the center of the fiber, and the y-axis gives the refractive index, defined by its difference relative to the index of the cladding of the fiber, in absolute difference (xcex94n) or in percentage difference (%xcex94). The definition of the relative difference between the index n1 and the index nc of the fiber may be written as follows:
%xcex94=100.(n12xe2x88x92nc2)/2n12
Given the small differences between the indices, this relative difference is substantially equal to the quantity (n1xe2x88x92nc)/nc; it is thus common to use the approximation %xcex94=xcex94n/nc, where nc is the refractive index of silica.
The index profile is thus said to be xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapezium-shapedxe2x80x9d or xe2x80x9ctriangularxe2x80x9d for curves representing the variation of refractive index as a function of radius that are respectively stepped, trapezium-shaped, or triangular. Such curves generally represent the ideal or reference profile of the fiber, the constraints involved in manufacturing the fiber often giving rise to a profile that is significantly different. By way of example, U.S. Pat. No. 5,659,649 shows a reference profile that is trapezium-with-ring shaped, and the different reference profile that is obtained on manufacturing the fiber.
Such single-mode fibers must also preferably have characteristics that correspond to the requirements both of cable-makers and of system designers: firstly, cable-makers look for good xe2x80x9ccablabilityxe2x80x9d, i.e. they want the fiber to be suitable for being included in a cable without inducing any or much additional attenuation. One solution to that problem consists in reducing the mode diameter of the fiber. Secondly, system-designers, desire large effective area, and a suitable value for the zero-dispersion wavelength xcex0. In addition, interest has recently arisen for xe2x80x9cNon-Zero Dispersion-Shifted Fiberxe2x80x9d (NZ-DSF) which is dispersion-shifted fiber in which chromatic dispersion at 1550 nm differs from zero.
Patent U.S. Pat. No. 5,327,516 describes a dispersion-shifted optical fiber in which it is proposed to reduce the non-linear effects by imparting a shallow chromatic dispersion gradient at the wavelengths in question, the value of the chromatic dispersion at said wavelengths generally lying in the range 1 ps/nmxc2x7km to 4 ps/nmxc2x7km.
Unfortunately, such a fiber still suffers from non-linear effects, such as four-wave mixing, that are considerable. That is why it appears to the applicant that it is desirable to have a dispersion-shifted fiber that minimizes non-linear effects.
The invention thus proposes a new type of dispersion-shifted fiber advantageously having losses in dB/km that are low, a chromatic dispersion gradient that is shallow, and a chromatic dispersion value at 1550 nm that is sufficiently high. Such a fiber makes it possible to minimize non-linear effects as compared with prior art NZ-DSFs.
More precisely, the invention provides a dispersion-shifted optical fiber comprising a core and cladding, with an index profile of shape consisting of a rectangular central portion surrounded by an annular portion of index lower than the index of the cladding, and then by the cladding, with a zero chromatic dispersion at a wavelength value xcex0 lying in the range 1380 nm to 1450 nm, and with chromatic dispersion at 1550 nm lying in the range 8 ps/nmxc2x7km to 12 ps/nmxc2x7km.
In an embodiment, the fiber has zero chromatic dispersion for a wavelength value xcex0 lying in the range 1400 nm to 1440 nm, and generally and in preferred manner approximately in the range 1400 nm to 1420 nm.
Advantageously, the fiber has an effective area of about 50 xcexcm2.
In an embodiment, the fiber has a cutoff wavelength shorter than 1310 nm.
In another embodiment, the fiber has a mode diameter at 1550 nm lying in the range 7.5 xcexcm to 8.5 xcexcm.
In yet another embodiment, the fiber has chromatic dispersion at 1550 nm lying in the range 9 ps/nmxc2x7km to 11 ps/nmxc2x7km, and generally and in preferred manner in the vicinity of 10 ps/nmxc2x7km.
Preferably, the fiber has a chromatic dispersion gradient of less than 0.06 ps/nm2xc2x7km at 1550 nm.
Preferably, the fiber has attenuation of less than 0.25 dB/km at 1550 nm.
Advantageously, said rectangular central portion has a refractive index that differs from the index of the cladding by a difference lying in the range 6xc3x9710xe2x88x923 to 9xc3x9710xe2x88x923.
Said rectangular central portion preferably extends to a radius lying in the range 2.8 xcexcm to 3.2 xcexcm.
In an embodiment, said annular portion has a refractive index that differs from the index of the cladding by a difference lying in the range xe2x88x920.3xc3x9710xe2x88x923 to xe2x88x921.3xc3x9710xe2x88x923.
Said annular portion advantageously extends to a radius of not less than six times the radius of said central portion.
The invention also provides the use of such a fiber in a wavelength division multiplexed system.